Stratified phase-separated composite having cross-linked polymeric layer

ABSTRACT

A stratified-phase-separated composite comprises a liquid layer, such as a liquid crystal layer, and a polymeric layer. In order to obtain a more complete stratification resulting in less liquid being present in the polymeric layer, the polymeric layer is crosslinked. The stratified composite is preferably obtained by photo-polymerization induced phase-separation of a phase-separable composition provided on a single substrate or dispersed between two opposite substrates.

[0001] The invention relates to a stratified phase-separated compositeand a method of manufacturing such a composite.

[0002] In Science vol.283 (1999) page 1903, Kumar et al. (see also U.S.Pat. No. 5,949, 508) disclose a phase-separated composite and a methodof manufacturing such. The known composite is manufactured by providing,between a pair of opposed substrates, a layer of a photo-polymerizableprepolymer dissolved in an organic liquid, in particular a liquidcrystal. The organic liquid and monomer are selected such that theliquid is poorly miscible with the photo-polymerized monomer. If soselected, phase-separation of the liquid and the photo-polymer takesplace during photo-polymerization, a process known in the art aspolymerization-induced phase separation (PIPS). In the composite ofKumar et al. the organic liquid is furthermore adapted to absorb the UVlight used for photo-polymerizing the monomer. Therefore, according toKumar et al, upon subjecting the layer to UV light, a light intensitygradient is set up in the layer in directions normal to the layer, thehighest intensity occurring at the side layer facing the UV lightsource. Since the rate at which photo-polymerization takes place scaleswith the intensity of UV light, photo-polymerization and thereforephase-separation preferentially takes place at the side of the layerfacing the light source. As a result the phase-separation takes place ina stratified manner, producing a composite comprising a predominantlypolymeric layer formed at the UV light source side and a predominantlyliquid layer at the side facing away from the UV light source.

[0003] A disadvantage of the known composite is that the stratificationis not complete. In particular, the polymeric layer comprises smallamounts of liquid crystal material. For many applications suchinclusions of LC material may be undesirable. For example, the liquidcrystal present in the polymeric layer may give rise to spuriousswitching effects and/or during the useful lifetime of the compositeliquid crystal material may migrate to and merge with the liquid layerso as to affect the properties of the liquid crystal layer, such as itsretardation or orientation.

[0004] It is an object of the invention, inter alia, to provide astratified phase-separated composite which does not have these drawbacksor at least to a lesser extent. In particular, it is an object toprovide a stratified-phase-separated composite which is well-stratified.In particular, the number of liquid molecules present in the polymericlayer (regardless the form in which such liquid molecules are present,ie dispersed as droplets or dissolved on a molecular scale) is to below.

[0005] In accordance with the invention, these objectives are achievedby a stratified phase-separated composite comprising a crosslinkedpolymeric layer and a liquid layer, the composite being obtainable bycrosslinking a layer of a crosslinkable, stratified-phase-separablecomposition comprising a crosslinkable material and a liquid.

[0006] The stratified-phase-separated composite in accordance with theinvention has a separate liquid layer and a chemically crosslinkedpolymeric layer. In contrast to polymer dispersed liquid crystals theliquid layer is a continuous layer. The composite in accordance with theinvention is well stratified in that only a small number of liquidcrystal molecules is present in the crosslinked polymeric layerindicating that the phase-separation is more complete. Upon crosslinkingof the crosslinkable composition, phase-separation occurs in astratified manner to produce a layer of a crosslinked polymer and alayer of liquid. A crosslinked polymer, in the art also referred to as anetwork polymer, has a polymer network extending in three dimensions,the network structure rendering the polymer relatively rigid compared tocorresponding linear chain polymers.

[0007] Although not wishing to be bound by any theory, it is believedthat, compared to linear chain polymers, crosslinked polymers areparticularly effective in squeezing out liquid molecules from thepolymeric layer being formed. Specifically, in the early stages of thestratification process it is observed that a polymeric layer is formedwhich extends almost across the entire layer ofstratified-phase-separable material. In the early stages this polymericlayer has no to very few crosslinks and is swollen with liquidmolecules. Upon further crosslinking, the number of crosslinksincreases, the polymer network becomes more dense and, as a result, thepolymeric layer contracts, its thickness becoming smaller, thussqueezing out the liquid molecules. As a result, the crosslinkedpolymeric layer contains less liquid material and the stratifiedphase-separation is more complete compared to a polymeric layer made ofa linear polymer.

[0008] Further advantages of crosslinked polymers are improvedtemperature and mechanical resistance and resistance to chemicals suchas solvents, the latter being convenient if the polymeric layer is toserve as a substrate for wet deposition of further layers. Moreover,being crosslinked, the polymer has a high strength and may, if used tosupport further layers or to serve as a substrate, be applied in thinnerlayers and still provide adequate support.

[0009] The composite is obtainable by stratification of a singlehomogenous layer. The stratification is by means of phase-separation. Inthe present invention, phase-separation is induced by crosslinking.Other methods of phase-separation are known in the art and, in order tooptimize the extent of phase-separation, crosslinking is used incombination with such other method, such other method being, forexample, solvent-induced phase-separation or thermally-inducedphase-separation. Dynamically stabilized or metastable mixtures ofnon-miscible liquid and crosslinkable material may also be used toinduce phase-separation. Preferred however is to use crosslinking incombination with polymerization-induced phase-separation (PIPS). PIPSmay thermally initiated or photo-initiated. Preferred isphoto-initiated, that is photo-polymerization-induced phase-separation.

[0010] Stratification, that is phase-separation in a stratified manner,may be brought about in various ways.

[0011] Stratification may be achieved by making use of differences insurface tension in particular differences in wetting. The composite isthen obtained by supplying the stratified-phase-separable composition ona substrate, the liquid having a spreading contact angle on thesubstrate and the liquid being capable of wetting the substratesignificantly better than the crosslinkable material being polymerized.When phase-separation is induced by means of crosslinking, optionally incombination with other phase-separation means described hereinabove, itis energetically favorable for the liquid to spread on the substrate andpush crosslinkable material (being polymerized) adjacent the substrateaway from the substrate thus forming a liquid layer directly adjacent tothat substrate and a polymeric layer on the other side.

[0012] Differential wetting may be employed using a single substrate butmay be used and even more effectively so if the composition is providedbetween two substrates.

[0013] Differences in wettability can be achieved by subjecting thesubstrate surface(s) to a wetting treatment such as plasma or UV ozonetreatment or apply a wetting layer or mix the stratified-phase-separablematerial with a wetting agent. Such layers, agents and treatments areall well known in the art.

[0014] An alternative method of stratification, one which may be appliedindependent of differential wetting but preferable is applied inconjunction with differential wetting comprises applying acrosslinkable, stratified-phase-separable composition which isphoto-polymerizable and which has absorbing means to set up a lightintensity gradient with respect to the light being used tophoto-polymerize. The composition may be rendered absorbing by using anabsorbing liquid or an absorbing photo-crosslinkable monomer or byadding a separate photo-polymerization dye.

[0015] For further details regarding the use of photo-polymerization toobtain stratified-phase-separated composites, in particular detailsrelating to dyes suitable for this purpose, such as a dye whichselectively accumulates in the polymeric layer or a photo-reactive dyewhich during photo-polymerization becomes chemically linked to themonomer being polymerized or a dye which is co-polymerizable with the(photo-crosslinkable) monomer, reference is made to a companion patentapplication, viz. an international application entitled “Stratifiedphase-separated composite comprising a photo-polymerization dye” filedon the same day by Applicant (Applicant's reference PHNL010919) claimingpriority of, inter alia, an European patent application havingapplication number 00204529.2.

[0016] The crosslinked polymer is obtained by crosslinking a compositioncomprising a crosslinkable material. By definition, the crosslinkablematerial is capable of forming a crosslinked polymer.

[0017] The crosslinkable material may be a (linear) crosslinkablepolymer, crosslinkable using high-energy actinic radiation such ase-beam or crosslinkable using a crosslinking agent such as thevulcanization of rubber using sulfur. The high-energy abstracts hydrogenatoms from the polymer chains, creating radicals on the polymer chains,which radical may then combine to form covalent bonds, each suchcovalent bond creating a crosslink of one single covalent bond.

[0018] In order to obtain stratified-phase-separable compositions whichhave good film forming properties and have a viscosity suitable foreffective, it is preferred that the crosslinkable material is acrosslinkable monomer or a crosslinkable monomer composition. Themonomer is generally a low molecular weight compound or in particular aprepolymer. A monomer or compound is crosslinkable (also referred to asa crosslinker or crosslinking agent) if it has a functionality of largerthan 2, meaning that it is capable of reacting with more than 2 othermolecules. Typically, crosslinks are formed by means of covalent bondsbut H-bridges may also be used to form crosslinks, H-bridged andcovalent crosslinks collectively being referred to as chemicalcrosslinks. In crosslinkable monomer compositions, preferably, thecrosslinkable compound is a crosslinkable monomer or is combined with amonomer, where monomer means a compound functionalized with apolymerizable group.

[0019] Suitable polymerizable monomers include monomers for performing apolycondensation, such as monomers for obtaining a polyether, apolyester or a polyamide, monomer for performing a cycloadditionreaction, a ring opening reaction, a polyaddition, a chainpolymerization or a step polymerization. Preferred are monomers forfree-radical chain polymerization.

[0020] Examples of preferred monomers, in particular forpolymerization-induced phase-separation, include a thiol-ene system ormonomer, an oxetane, an epoxide, a vinylene, a vinylether, an acrylate,a methacrylate or a cinnamate monomer. With the exception of thecinnamate group which has a functionality of 1, the polymerizable groupof these preferred monomers have a functionality of 2 meaning thatmonomers having one polymerizable group, that is mono-(meth)acrylates,mono-epoxides, mono-vinylethers, mono-oxetane, mono-vinylenes, andthiol-ene monomers, suffice to form a linear polymer. To obtaincrosslinked polymers, monomers comprising two or more polymerizablegroups each having a functionality of at least 2 may be used, such asdi(meth)acrylates, di-vinlyethers, di-oxetanes, di-vinylenes,di-epoxides, or thiol-ene systems comprising trithiols or di-enes ordi-enes provided with a mercapto group or ene-functionalized dithiols orhigher homologues of such monomers, all of which are known in the artper se. Crosslinked cinnamates require monomers carrying at least threecinnamate groups. Particularly preferred are combinations of monomershaving one polymerizable group and monomers having two or morepolymerizable groups as they allow the degree of cross-linking to befreely selected. Hydrolytically condensible organo-metallic compoundssuch silicon alkoxides may also be used to obtain crosslinked polymers.

[0021] The stratified-phase-separable material may comprise just onetype of monomer to produce a homopolymer but generally it will containmore than one type to obtain copolymers, the term copolymers includingterpolymers or higher homologues. The monomer may itself be a polymer,also referred to as a prepolymer, which is further polymerized and/orcrosslinked to obtain the crosslinked polymer. Differentnon-co-polymerizable monomers may also be used to obtain a polymer blendwhich may or may not be phase-separated.

[0022] Combinations of liquid crystal and monomers which may be suitablecandidates for use in the stratified-phase-separable compositions inaccordance with the invention are those used to manufacture polymerdispersed liquid crystals.

[0023] The stratification, in particular the amount of liquid beingtrapped in the crosslinked polymeric layer, will depend on the degree ofcrosslinking. Generally, if the degree of crosslinking is very low thepolymer will substantially behave as a linear polymer and thestratification process will be improved to a small extent only. On theother hand, if the degree of crosslinking is very high the polymernetwork is formed very quickly, leaving very little time for the liquidto migrate to the liquid layer being formed. This will lead to liquidbeing trapped in the polymeric layer. It will also be understood bythose skilled in the art that the required degree of crosslinking toobtain optimal stratification depends of the particular combination ofliquid and crosslinkable material used and the rate at which thestratification is carried out.

[0024] In the context of the present invention, the degree ofcrosslinking is defined in terms of the crosslink density, crosslinkdensity being expressed as the number of crosslinks in moles per 1000 gof polymer.

[0025] In case of a stratified-phase-separated composite comprising acrosslinked polymer obtained by subjecting a linear polymer tohigh-energy radiation, such as e-beam, to abstract hydrogen atoms thusforming radicals on the linear polymer chains, which radicals then reactwith each other to form a single covalent bond, the single covalent bondbeing the crosslink, the crosslink density is, preferably, at leastabout 0.1 to 0.5 and at most about 10, or, better, 5, or preferably 2.5.If the crosslink density is higher than the maximum, the improvement instratification is relatively small whereas the time to manufacture thecomposite as well as the radiation dose continues to increase, thusincreasing the risk of damage being done to the composite or thesubstrate on which is manufactured by the high-energy radiation. If thecrosslink density is below the lower limit, improvement ofstratification is impaired.

[0026] In case of a stratified-phase-separated composite obtained from astratified-phase-separable composition wherein the crosslinkablematerial is a crosslinkable monomer or a crosslinkable monomercomposition, the crosslink density of the polymer may be calculatedusing the functionality of the monomers. Specifically, a crosslinkermolecule having functionality f contributes f-2 crosslinks. Assuming allcrosslinkers fully react to form the crosslinked polymer, the crosslinkdensity is calculated as 1000*Σ_(i)x_(i)/M_(i)*(f₁-2), where x_(i) isthe weight fraction of crosslinker i relative to the total monomerweight in the stratified-phase-separable composition (Σ_(i)x_(i)=1),M_(i) is the molar mass of crosslinker i and f_(i) is the functionalityof crosslinker i.

[0027] Generally, the crosslink density will be between about 0.1 toabout 10 moles crosslinks per 1000 g of polymer. Preferably, however thelower limit is about 0.15, or better about 0.25. In selected cases, theupper limit is preferably about 6.0 or even better about 2.5. In apreferred embodiment, the crosslink density is in the range from 0.15 to2.5.

[0028] If the crosslink density is below the lower limit, improvementsin stratification may be insufficient, whereas if the upper limit isexceeded the network formation may proceed too rapid, leading tosubstantial amounts of liquid being trapped in the crosslinked layer.Moreover, if the degree of crosslinking is high it is difficult toobtain a full conversion of the polymerizable groups.

[0029] If the crosslinkable composition contains a combination ofmonomers which are as such not crosslinkable and crosslinkable monomers,it may be useful to express the degree of crosslinking as the molefraction of crosslinkable monomer of the total monomer amount.Generally, the molar fraction may vary from 0.01 to 0.99. Typically,however the molar fraction is 0.75 or less, or more specific, 0.50 orless, or even 0.25 or less.

[0030] In the art, different methods to crosslink in casu polymerize acrosslinkable monomer or monomer composition are known. In a firstmethod, crosslinking is induced by heating the crosslinkablecomposition, in the art also known as thermosetting where heating isperformed by conventional means such as exposure to infrared (IR)radiation. Alternative to or in combination with heating, crosslinkingmay be induced by actinic radiation, in the art also known asphotosetting, photo-polymerization or photo-crosslinking. In both casescrosslinking may be facilitated by an initiator, such initiators beingknown per se. In case crosslinking involves free radical polymerization,the initiator is a compound which upon applying heat or radiationproduces free radicals. Actinic radiation includes e-beam radiation,gamma radiation and electromagnetic radiation such as X-rays, visiblelight and UV light.

[0031] There is no limit on the type of liquid which may be used for thecomposite in accordance with the invention other than it should be ableto form a liquid layer by means of stratified phase-separation. Examplesinclude inorganic liquids, such as water or water-based liquids, andorganic liquids.

[0032] In a particularly suitable embodiment, the liquid is a liquidcrystal.

[0033] Suitable liquid crystals include those capable of forming aplanar, a homeotropic, a twisted or splay orientation. The orientationcan also be uniaxial or biaxial. Any LC phase may be suitably used, suchas nematic, twisted nematic, cholesteric, discotic, smectic A and C,ferroelectric, flexoelectric and the like. The liquid crystal layer maybe partitioned into a number of distinct domains, such as sub-pixeldomains, each domain having a different anisotropic orientation. Inparticular, the difference in orientation may be limited to a differencein the orientation of the director(s) while the LC phase is the same.

[0034] The liquid may also be a polymerizable or polymerized liquid. Thelatter, being a solid, is of particular advantage in applications wherethe fluid nature of the liquid is of no relevance for the function to beachieved.

[0035] The thickness of the liquid layer will depend on the particularapplication sought but generally will vary from about 0.1 μm to about 1mm. In case the liquid layer is a liquid crystal layer the thicknesswill typically be 0.5 μm to 10 μm or more particular, 1 μm to 6 μm. Thethickness of the polymeric layer will depend on its function in thecomposite. If the polymeric layer as such does not have to provide thenecessary protection and/or ruggedness, resistance to tear and othermechanical forces, but has to be capable of providing a substratesurface for the provision of subsequent layers such as layers which doprovide the necessary ruggedness and/or mechanical integrity thepolymeric layer can be relatively thin, that is at least about 0.1 to0.2 μm and at most about 5 to 10 μm. On the other hand, if the polymericlayer is to provide a significant contribution to the mechanicalintegrity of the composite, a thicker polymeric layer is preferable,typically larger than 5 μm. Since the time required to phase-separateincreases as the combined thickness of liquid layer and polymeric layerincreases it is generally desirable to keep the combined thickness aslow as possible. Typically, the combined thickness would be less than100 μm or more particular less than 50 μm.

[0036] The specific relative amounts of liquid and polymerizablematerial in a stratified-phase-separable composition in accordance withthe invention will depend on the desired ratio of liquid to polymericlayer thickness but generally will be between about 1 and 99 percent byweight. Phase-separation is more easily facilitated if the relativeamount of liquid or monomer ranges between 5 and 90 wt % or betterbetween 10 and 80 wt %. The relative amount of photo-polymerization dye,if present and added as a separate component, is determined by thedesired light intensity gradient. Typically, the amount will be lessthan 20 wt % or even less than 10 wt % of the total weight of thestratified-phase-separable composition.

[0037] A substrate may be used to confine the liquid layer during use ofthe composite and/or to provide a surface on which a layer ofphase-separable material may be provided. Suitable substrates includeglass and plastic but also metal mirror coated or silicon substratesoptionally comprising integrated circuits manufactured using CMOStechnology. If the composite is used for a transmissive opticalapplication the single substrate is to be transparent. The composite inaccordance with the invention may in particular be combined with aflexible substrate, such as a foldable substrate. In order to facilitateroll to roll manufacturing of the composite a wrappable substrate may beused. Suitable materials for flexible, foldable and/or wrappable singlesubstrates include polymer films and sheets, metal foils and coatedpaper or laminates thereof.

[0038] In a particular embodiment, the stratified-phase-separatedcomposite comprises a liquid layer dispersed between a first and secondpolymeric layer, the composite being obtained by photo-crosslinking astratified-phase-separated composition. Such a composite may bemanufactured by irradiating a layer of photo-polymerizablestratified-phase-separable material having a photo-polymerization dyefrom both sides such a light intensity gradient is set up which has aminimum light intensity in the middle and which increases towards theouter surfaces. Such irradiation requires the use of a substratetransparent for the radiation used.

[0039] In case the liquid layer is a liquid crystal layer which is to beanisotropically oriented it is convenient—as is well known in the art—toprovide the substrate with an alignment layer or provide the substratesurface with some other alignment inducing means. The type of alignmentlayer used is as such not critical provided the necessary wettabilityconditions are satisfied. Any conventional alignment means, rubbed,photo-aligned or otherwise, may be used including in particular apolyvinylalocohol alignment layer or a polyimide layer. The alignmentlayer may also be used means to achieve a differential wetting of theliquid and polymeric/polymerizable material with respect to thesubstrate and thus bring about a well-defined stratification.

[0040] Combining a composite in accordance with the invention with asubstrate during use and/or manufacture at least confines the liquid indirections normal to the liquid layer. In order to prevent liquid fromleaking away from the composite, the composite may be packaged.Alternatively, by providing the substrate with a recess or with ridgesan enclosure adapted to contain phase-separable material may be formed.After phase-separation, the polymeric layer caps the enclosure and itsperimeter is attached to the side-walls of the enclosure thus obtaininga liquid tight container. The enclosure may be formed in any convenientmanner for example, in the case of a plastic substrate, by injectionmolding. In a particular embodiment, ridges forming an enclosure may beobtained from the photo-polymerizable phase-separable material bypattern-wise photo-polymerization, eg by means of a mask having apattern outlining the ridges to be formed.

[0041] In order to improve the mechanical integrity and stability of thestratified-phase-separated composite and/or maintain a well-definedliquid crystal layer thickness, the liquid crystal layer may compriseand/or may be partitioned by connecting (supporting) members whichconnect the substrate to the polymeric layer. Thus, the thickness of theconnecting members exceeds the thickness of the liquid crystal layer.The connecting members may be conventional spacers which are partiallyembedded in the polymeric layer, or a relief structure patternphoto-lithographically provided on the substrate before thestratified-phase-separated composite is formed using for example aphoto-resist. In a very advantageous embodiment, the connecting membersare formed by pattern-wise photo-polymerizing photo-polymerizablestratified-phase-separable composition, e.g. by means of a mask. Thepattern-wise photo-polymerization is conveniently performed before orsimultaneous to the flood-exposure required to form the phase-separatedpolymeric and liquid layer.

[0042] The composite in accordance with the invention may be used for avariety of applications. In its broadest sense it may be used for anyapplication involving a liquid. A general application if combined with asubstrate, is a liquid-tight packaged liquid for containing thin (about0.1 μm to about 1 mm) films of liquid of large surface area (about 1 cm²to about 1 m² or more). The composites in accordance with the inventionallow such liquid-filled packages to be formed quickly and in an easymanner. Filling large area thin containers with liquid in thetraditional manner is cumbersome. If the liquid is selected to bepolymerizable, the composite in accordance with the invention may alsobe used for solid-state applications.

[0043] An important class of applications are optical andelectro-optical applications in particular when the liquid is liquidcrystal. In particular if photo-polymerization is performedpattern-wise, microlens arrays, gratings and structures can bemanufactured.

[0044] In a preferred embodiment, the invention relates to a displaydevice comprising a composite in accordance with the invention.

[0045] The composite in accordance with the invention may comprise aliquid crystal layer which is switchable between a first and secondstate, the first and second state having different optical propertiessuch as a difference in polarization selectivity. Thus, the composite inaccordance with the invention may be used in a LC display device. Inprinciple, there is no limitation on the LC effect and device. Howeverin a preferred embodiment an in-plane switching arrangement is usedbecause the required electrodes may all be formed on the substrate usedto support the stratified-phase-separated composite. As the compositesmay be manufactured in a continuous process rather than a batch processthe composites may be of particular advantage in roll-to-rollmanufactured displays.

[0046] The invention also relates to a method of manufacturing astratified-phase-separated composite in accordance with the invention.In a first embodiment, the method comprises:

[0047] providing a supporting substrate; applying, on the substrate, alayer of crosslinkable, stratified-phase-separable compositioncomprising a crosslinkable material and a liquid;

[0048] crosslinking the layer of crosslinkable,stratified-phase-separable composition thus formed to inducephase-separation into a stratified phase-separated composite comprisinga liquid layer and a crosslinked polymeric layer.

[0049] In a broad sense, the method of this embodiment, which may alsobe referred to as a single substrate method, provides an alternativemethod of forming a packaged liquid layer. The method is of particularuse in case the liquid layer to be packaged is thin, say about 0.1 μm to1 mm, is of large surface area, typically form about 0.1 cm² to about 1m² or more and/or has to have a uniform well-defined thickness to bekept constant during use of the packaged liquid layer. Packaging thinand large area liquid layers by filling a thin and large area containerwith liquid is cumbersome. The method in accordance with this embodimentis suitable for a bottom-up process in which layers are stacked on topof the other. The method may in particular be combined with theprovision of further layers by wet deposition methods such as coatingand printing methods. The method may be performed in a batch process butalso in a continuous process, in particular the method may be used in aroll-to-roll manufacturing process thus allowing cost-effectivemass-production.

[0050] In a preferred embodiment of the single substrate method, thecrosslinkable, stratified-phase-separable composition isphoto-polymerizable (photo-crosslinkable) and contains aphoto-polymerization dye which selectively accumulates in the polymericlayer being formed. (Meth)acrylate monomers are preferred monomers foruse in the single substrate manner.

[0051] A plurality of single substrate composites obtainable from thesingle method may be stacked to form a stack of single substratestratified-phase-separated composites. Such a stack of composites may,for example, be used to obtain a multi or even full color display inwhich the active LC layers are stacked one on top the other to gain afactor of three in active display area. By using a single substrate toform a stacked display instead of a double substrate the distancebetween the active layers can be reduced to twice the thickness of thepolymeric layer which may as low as 5 to 10 μm. In this way parallaxeffects prominent in conventional stacked displays are significantlyreduced.

[0052] In one embodiment, the single substrate method is repeated anumber of times in succession where the single substrate stack of aprevious time is used as the single substrate for a next singlesubstrate step. Alternatively, two single substrate composites may beprepared separately and then attached to each via their polymericlayers. In this embodiment, the electrodes may be provided on bothsingle substrates to form a single sandwich electrode arrangement oreach substrate may be provided with in-plane switching electrodes torender both liquid layers (independently) switchable.

[0053] Another embodiment of the method in accordance with the inventioncomprises:

[0054] providing a cell adapted to contain a layer of a crosslinkable,stratified-phase-separable composition;

[0055] filling the cell with crosslinkable, stratified-phase-separablecomposition comprising a crosslinkable material and a liquid layer;

[0056] crosslinking the layer of crosslinkable,stratified-phase-separable composition thus formed to inducephase-separation into a stratified phase-separated composite comprisinga liquid layer and a crosslinked polymeric layer.

[0057] This method, also referred to as the double substrate method, maybe useful for example when very thin polymer layers are to bemanufactured. Such a thin polymeric layer may be desirable if thecomposite forms part of an LC cell in which the liquid layer of thecomposite is the active LC layer. In case the LC layer is to be renderedswitchable by sandwiching the composite between two opposed substrateseach provided with electrodes, the polymeric layer is preferably thin toreduce capacitance.

[0058] Further details regarding the single and double substrate methodwith respect to phase-separation, stratification and cross-linking havebeen described hereinabove.

[0059] These and other aspects of the invention will be apparent fromand elucidated with reference to the examples described hereinafter.

[0060] In the drawings:

[0061] The sole FIGURE shows, schematically, in a cross-sectional view,a cell comprising a stratified phase-separated composite in accordancewith the invention.

EXAMPLE 1

[0062] A stratified-phase-separated composite in accordance with theinvention is manufactured using a method in accordance with theinvention as follows:

[0063] A stratified-phase-separable composition is prepared having thefollowing composition:

[0064] 50.0 wt % liquid crystal E7,

[0065] 44.5 wt % photo-polymerizable isobomylmethacrylate (formula A1),

[0066] 0.5 wt % photo-initiator (formula A2), and

[0067] 5.0 wt % of hexanedioldimethacrylate, acronymed HDODMA (formulaA6)

[0068] Liquid crystal E7 is marketed by Merck and comprises a mixture ofcyanobiphenyls and cyanoterphenyls. The photo-initiator A2 is marketedunder the trade name Irgacure 651 by Ciba Geigy. The dimethacrylateHDODMA can be purchased from Aldrich.

[0069] The liquid crystal E7 has a significant absorption in the rangeof 300 to 350 nm. The photo-initiator used to initiate thephoto-polymerization also absorbs in this wavelength range, so theliquid crystal E7 is a photo-polymerization dye, that is a dye whichabsorbs at least partially the radiation used to photo-polymerize.

[0070] Being a monomethacrylate, the monomer A1 is (photo)-polymerizablebut not crosslinkable.

[0071] Being a dimethacrylate, a monomer having functionality 4, hencecapable of reacting with four other monomers, the HDODMA monomer A6 isphoto-polymerizable and crosslinkable. The monomer A6 does not absorb inthe wavelength range 300 nm to 350 nm used to photo-polymerize.

[0072] The stratified-phase-separated composite obtained byphoto-polymerizing this stratified-phase-separable composition comprisesa crosslinked polymer. The crosslink density, expressed in terms of thenumber of crosslinks in moles per 1000 g of polymer, assuming alldimethacrylate monomers have fully reacted, is 1000*( (5/(44.5+5)/M_(A6))*(4 −2)) with M_(A6)=254, the molecular weight of A6, thecrosslink density is 0.8.

[0073] The sole FIGURE shows, schematically, in a cross-sectional view,a cell comprising a stratified phase-separated composite in accordancewith the invention.

[0074] A transparent cell 41, also referred to as a double substratecell, is made by positioning two glass substrates 42 and 43, thesubstrate 42 carrying a rubbed polyimide alignment layer 48 (AL1051 ofJSR) opposite one another at a distance of 7 μm by means of spacers andsubsequently gluing the substrates 42 and 43 together along the edgesleaving a small opening for filling. The cell thus made is filled, at50° C., by capillary action with a quantity of thestratified-phase-separable composition. The filled cell is, with thesubstrate 43 being arranged closest to a UV radiation source (PhilipsTL-08, 0.1 mW/cm2), exposed to UV-light for 60 minutes at a temperatureof 50° C. and then cooled down to room temperature. During exposure, dueto the absorption in the layer of phase-separable composition, a lightintensity gradient for wavelengths of 300-350 nm and normal to thesubstrates is established. The light intensity in the layer beinghighest at the side closest to the UV source, polymerization andcross-linking selectively occurs nearest to the substrate 43. The(partially) polymerized monomers formed by the UV exposure are notmiscible with the liquid crystal material E7 and thus phase separatesfrom the liquid crystal material. Under the influence of the lightintensity gradient set up by the blanket exposure, the phase separationproceeds in a stratified manner to form a stratum of crosslinked polymeron top of a stratum of liquid crystal E7, the liquid crystal stratumbeing formed closest to the substrate 42. Monomeric material isselectively depleted nearest to the substrate 43, more in particular atthe interface of the liquid crystal layer and polymer layer beingformed, as a result of which a diffusion gradient in monomer A1 and A6concentration is set up as a result of which monomer is continuously fedto the liquid crystal layer/polymer layer interface which allows thestratification process to proceed further. In the early stages of thestratification when the polymer formed comprises no to very fewcrosslinks, the polymeric layer being formed substantially extendsacross the entire thickness of the layer. Upon further polymerization,the the number of crosslinks increases, the polymer network becomes moredense and the polymeric layer contracts to form a thinner layer nearestthe substrate 43. Because of the contraction liquid crystal moleculeswhich are still present in the polymeric layer being formed areeffectively squeezed out. Finally, the photo-polymerization inducedphase separation produces a double substrate cell comprising astratified-phase-separated composite 47 comprising a crosslinkedpolymeric layer 46 and a liquid crystal layer 44 of E7.

[0075] From a stratified-phase-separated composite in accordance withthe invention thus manufactured, the liquid crystal material isrecovered and its clearing temperature is measured. The measuredclearing temperature is the same as that of virgin liquid crystal E7,which demonstrates that the liquid crystal layer 44 of the stratifiedphase-separated composite 47 essentially consists of liquid crystal E7.

[0076] Visual inspection using the unaided eye shows that thestratified-phase-separated composite does not scatter light inreflection or in transmission.

[0077] When a double substrate cell comprising stratified composite thusmanufactured is examined under a polarization microscope equipped withcrossed polarizers, the liquid crystal layer 44 of the composite 47 isfound to be birefringent. In particular, with thestratified-phase-separated composite arranged between two crossedpolarizers and aligned such that the rubbing direction of the polyimidelayer 48 makes an angle of 45° with the polar axis of each of the twopolars, the cell appears bright if illuminated from behind, whereas ifthe rubbing direction is aligned with the polar axis of one of thepolarizers the cell appears dark, indicating that the liquid crystallayer LC molecules have a more or less uniaxial alignment. Under 45° thecell presents a relatively homogeneously bright image. No dark spots areobserved. Dark spots occur at locations where non-birefringent materialis present. In particular, because the crosslinked polymer is notbirefringent, it is thus ascertained that the polymeric layer does nothave polymeric protrusions which extend from the polymeric layer all theway across the thickness of the liquid crystal layer.

[0078] In order to assess the extent to which thestratified-phase-separation has occurred, in particular to assess theamount of liquid crystal material present in the crosslinked polymericlayer, the retardation (dΔn) of the stratified-phase-separated compositeis measured. Since the crosslinked polymer is not birefringent and anyliquid crystal molecules which may be present in the crosslinkedpolymeric layer are not oriented on a macroscopic scale, the measuredbirefringence is expected to equal dΔn, where d is the thickness of theliquid crystal layer 44 and Δn is the birefringence of the liquidcrystal layer 44 at unit thickness. The retardation is measured to be136 nm.

Comparative Example 1 (Not in Accordance with the Invention)

[0079] Example 1 is repeated with the difference that the amount ofcrosslinkable monomer A6 is replaced with non-crosslinkable monomer A1.In particular the composition used in this example is:

[0080] 50 wt % liquid crystal E7,

[0081] 49.5 wt % photo-polymerizable isobomylmethacrylate (formula A1),

[0082] 0.5 wt % photo-initiator (formula A2),

[0083] This composition has no monomers having a functionality of morethan 2, is therefore not crosslinkable. The crosslink density is 0.0.

[0084] Polymerizing a layer of this composition formed in a doublesubstrate cell as shown in FIG. 1, results in astratified-phase-separated composite comprising a liquid layer and apolymeric layer, both the liquid layer and polymeric layer beingcontinuous. Since the polymeric layer is not crosslinked, the compositeis not in accordance with the invention.

[0085] The clearing temperature of E7 liquid crystal removed from thecomposite is the same as that of virgin E7 liquid crystal indicatingthat the liquid crystal layer substantially consists of liquid crystalmaterial.

[0086] Examination of the composite arranged between crossed polarizersunder a polarization microscope as described in detail in example 1shows the liquid crystal layer to be uniaxially aligned.

[0087] Visual inspection using the unaided eye shows some scattering oflight indicating that the liquid layer/polymeric layer interface is lesssmooth than in the composite in accordance with the invention of Example1.

[0088] The retardation dΔn is measured to be 62 mn.

[0089] Comparing this retardation with the retardation of Example 1, 136nm, the retardation of this Comparative example 1 is significantlylower. Assuming Δn is at least not smaller, this indicates that theliquid layer of Example 1 is thicker than in this Comparative example 1.Since the amount of liquid crystal used in both examples is the same,the thicker LC layer means that less liquid crystal molecules arepresent in the crosslinked polymeric layer.

[0090] Example 1 and Comparative example 1 together demonstrate that thephase separation is more complete, in the sense that less liquid crystalmolecules are present in the polymeric layer, if the polymeric layer iscrosslinked.

EXAMPLE 2

[0091] Example 1 is repeated with the difference that astratified-phase-separable composition is prepared having the followingcomposition:

[0092] 50.0 wt % liquid crystal E7,

[0093] 43.5 wt % photo-polymerizable isobornylmethacrylate (formula A1),

[0094] 0.5 wt % photo-initiator (formula A2),

[0095] 5.0 wt % photo-polymerizable stilbene monoacrylate dye (formulaA5), and

[0096] 1.0 wt % of hexanedioldimethacrylate, acronymed HDODMA (formulaA6)

[0097] The dye A5 absorbs in the wavelength range being used tophoto-polymerize the stratified-phase-separable composition and themolecular weight of the monomethacrylate A5 (M_(A5)) is 363.

[0098] The crosslink density, expressed in terms of the number ofcrosslinks in moles per 1000 g of polymer, assuming all dimethacrylatemonomers have fully reacted, is 1000*((1/49.5)/254)(4-2)=0.16.

[0099] The degree of crosslinking, expressed as the molar ratio ofcross-linkable monomer to non-crosslinkable monomer, is 1:47.

[0100] The composition thus prepared is processed into a cell comprisinga stratified-phase-separated composite in accordance with the inventionin the manner described in Example 1.

[0101] From a stratified-phase-separated composite in accordance withthe invention thus manufactured, the liquid crystal material isrecovered and its clearing temperature is measured. The measuredclearing temperature is the same as that of virgin liquid crystal E7,which demonstrates that the liquid crystal layer 44 of the stratifiedphase-separated composite 47 essentially consists of liquid crystal E7.

[0102] Visual inspection using the unaided eye shows that thestratified-phase-separated composite does not scatter light inreflection transmission.

[0103] When a double substrate cell comprising stratified composite thusmanufactured is examined under a polarization microscope equipped withcrossed polarizers, the liquid crystal layer 44 of the composite 47 isfound to be birefringent.

[0104] In particular, with the stratified-phase-separated compositearranged between two crossed polarizers and aligned such that therubbing direction of the polyimide layer 48 makes an angle of 45° withthe polar axis of each of the two polars, the cell appears bright ifilluminated from behind whereas if the rubbing direction is aligned withthe polar axis of one of the polarizers the cell appears dark,indicating that the liquid crystal layer LC molecules have a more orless uniaxial alignment. Under 45° the cell presents a relativelyhomogeneously bright image. No dark spots are observed. Dark spots occurat locations where non-birefringent material is present. In particular,it is thus ascertained that the polymeric layer does not have polymericprotrusions which extend from the polymeric layer all the way across thethickness of the liquid crystal layer.

[0105] The retardation of the stratified-phase-separated composite ismeasured to be 94 nm. This number is not necessarily directly comparableto the retardation of the previous examples as the birefringence Δn maybe different due to a slightly different orientation.

EXAMPLE 3

[0106] Example 2 is repeated except that the amount of cross-linkablemonomer A6 is increased at the expense of non-crosslinkable monomer A1.

[0107] Specifically the stratified-phase-separable composition is:

[0108] 50.0 wt % liquid crystal E7,

[0109] 37.0 wt % photo-polymerizable isobomylmethacrylate (formula A1),

[0110] 0.5 wt % photo-initiator (formula A2),

[0111] 5.0 wt % photo-polymerizable stilbene monoacrylate dye (formulaA5), and

[0112] 7.5 wt % of hexanedioldimethacrylate, acronymed HDODMA (formulaA6)

[0113] The crosslink density, expressed in terms of the number ofcrosslinks in moles per 1000 g of polymer, assuming all dimethacrylatemonomers have fully reacted, is 1000*((7.5/49.5)/254)(4-2)=1.19.

[0114] The molar degree of crosslinking, expressed as the molar ratio ofcross-linkable monomer to non-crosslinkable monomer, is(7.5/254):(37/223 +5/363)=1:6.08.

[0115] The stratified-phase-separated composite obtained from thiscomposition by manufacturing a cell as in example 2 has similarproperties as the composite of example 2. The retardation however ismeasured to be 187 nm.

[0116] Comparing the retardation of this Example 3, 187 nm, to that ofExample 2, 94 nm, shows that the extent of stratification, in the senseof the amount of liquid crystal being trapped in the crosslinkedpolymeric layer, depends on the crosslink density. In particular, thestratification is more complete if the crosslink density is increased.

1. A stratified-phase-separated composite comprising a crosslinkedpolymeric layer and a liquid layer, the composite being obtainable bycrosslinking a layer of a crosslinkable, stratified-phase-separablecomposition comprising a crosslinkable material and a liquid.
 2. Acomposite as claimed in claim 1, wherein the polymeric layer of thecomposite has a crosslink density, expressed as the number of crosslinksin moles per 1000 g of polymer, is in the range from 0.15 to 2.5.
 3. Acomposite as claimed in any one of the claims 1 or 2, wherein the liquidis a liquid crystal.
 4. A display device comprising a composite asclaimed in any one of the claims 1 to
 3. 5. A method of manufacturing astratified phase-separated composite comprising a crosslinked polymericlayer and a liquid layer, the method comprising: providing a supportingsubstrate; applying, on the substrate, a layer of crosslinkable,stratified-phase-separable composition comprising a crosslinkablematerial and a liquid; crosslinking the layer of crosslinkable,stratified-phase-separable composition thus formed to inducephase-separation into a stratified phase-separated composite comprisinga liquid layer and a crosslinked polymeric layer.
 6. A method ofmanufacturing a stratified phase-separated composite comprising acrosslinked polymeric layer and a liquid layer, the method comprising:providing a cell adapted to contain a layer of a crosslinkable,stratified-phase-separable composition; filling the cell withcrosslinkable, stratified-phase-separable composition comprising acrosslinkable material and a liquid layer; crosslinking the layer ofcrosslinkable, stratified-phase-separable composition thus formed toinduce phase-separation into a stratified phase-separated compositecomprising a liquid layer and a crosslinked polymeric layer.